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Small subgroup attack in Mozilla NSS

tl;dr While the TLS servers attacks has been pretty much studied and fixed (see e.g. https://www.secure-resumption.com/ and https://weakdh.org/) the situation with the TLS clients is (was) not ideal and can be improved. Here I report a Small subgroup attack for TLS clients that I performed against various browsers and reported.

Whoever reads this blog is used to read about OAuth .
For once (and maybe more in the future) let's hijack the usual topic and let's talk about my new "passion" : TLS in particular Diffie–Hellman (DH from now on).

Now, before to start I need to clarify one thing IANAC (I am not a cryptographer) so I might likely end up writing a bunch of mistakes in this blog post...

When using TLS_DHE_RSA_WITH_AES_128_CBC_SHA Firefox/Chrome doesn't accept degenerate public key of value 0,1 and -1 since this key lead to pms that is {0,1, -1}.
This (the -1 case) is probably a consequence of CVE-2014-1491 (raised as part of the Triple Handshake Attack ).

Of course the "worse" one is 1 and happens to be 1 time out of 4 (according to Adam Langley though "here's nothing special about sending an odd DH value, it could equally well make its DH private key equal to 42"). So not big deal :(

Just for the record even the easier suggestion given in [1] aka "Make sure that g^x,g^y and g^xy do not equal to 1"

tl;dr I found a severe issue in the Slack's SAML implementation that allowed me to bypass the authentication. This has now been solved by Slack.
Introduction
IMHO the rule #1 of any bug hunter (note I do not consider myself one of them since I do this really sporadically) is to have a good RSS feed list. In the course of the last years I built a pretty decent one and I try to follow other security experts trying to "steal" some useful tricks. There are many experts in different fields of the security panorama and too many to quote them here (maybe another post). But one of the leading expert (that I follow) on SAML is by far Ioannis Kakavas. Indeed he was able in the last years to find serious vulnerability in the SAML implementation of Microsoft and Github. Usually I am more an "OAuth guy" but since both, SAML and OAuth, are nothing else that grandchildren of Kerberos learning SAML has been in my todo list for long time. The Github incident gave me the final…

then after that the resource owner has authorized the client the authorization server redirects the resource owner back to the client with an authorization code:
Then the OAuth dance continues....
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